Executing client script before page load

[Eric Livingston]

I'm working on a custom server control that inherits from textbox. I want to
use the control to display a datetime object using the appropriate timezone
offset for the client.

Using ClientScriptManager.RegisterHiddenField, I've got a place to store the
value returned by the js getTimezoneOffset. However, I need to execute the
client script as early as possible in the page lifecycle to retrieve that
value so that it is available in the code behind by the time the page load
event fires.

Any suggestions?

Thanks!